The present invention relates generally to color cathode ray tube displays formed either by conventional shadowmask technology or by optical combination of different color display fields in a projection device. More particularly, the invention relates to a memory mapped deflection correction system for use in such cathode ray tube displays.
Certain aircraft cockpit displays use a delta-gun shadow mask cathode ray tube (CRT). The delta-gun CRT has the best performance of any shadow-mask based color CRT because it operates with a uniform deflection field which preserves the round spot and provides the largest possible focus barrel within a given neck diameter. These characteristics result in nearly equal resolution on all axes of the display. That is, lines written on different angles are all nearly equal in width. Because of physical restrictions characteristic of such CRTs, none of the electron guns fires through the center of deflection. Instead they are spaced equally around the geometric central axis of the CRT, one gun every 120 degrees. The consequence of not passing through the center of deflection is that during deflection, each of the three electron guns encounters a slightly different deflection. This difference in deflection introduces error in the ability to converge the CRT beams. Second order effects cause this error to be different in different quadrants of the CRT and to vary from one CRT assembly to another. This misconvergence effect is apparent on the CRT whenever a secondary color is drawn (secondary colors are any color produced by a combination of primary colors).
Suppose, for example, that a yellow line is being drawn. Yellow is produced by the superposition of red and green primary luminance. If the red and green lines perfectly overlay, and each line is about 0.021 inches wide, then a yellow line which is 0.021 inches wide results. Because of misconvergence, these lines may offset, for example, by about 0.002 inches. The resultant line, viewed from a distance of say 19 inches, is a 0.023 inches wide yellow line. No color fringing would be observed for this amount of misconvergence but the resolution capability of the display on secondary colors would be degraded. For larger convergence errors, e.g., 0.018 inches, there is loss of resolution and color fringing. Color fringing is a breakup of the chromatic characteristic of the line into red, yellow, and green components. For some applications, misconvergence anywhere on the CRT cannot exceed 0.010 inches. This is an amount which, in one analog convergence system, requires 52 potentiometers to allow appropriate adjustment of the amounts of correction waveforms in each of 4 quadrants of the CRT display. A description of a method of doing this type of analog adjustment is contained in U.S. Pat. No. 4,524,307.
Such analog approaches consume space, power and parts, and reduce the reliability of the display. Such disadvantages limit the attractiveness of the delta gun CRT when compared, for example, to an in-line CRT. Except for the convergence problem, delta gun technology would be the clear choice for most applications based on its performance and simplicity. In addition to the problems inherent after the display is converged (but not perfectly converged), there are the problems associated with accomplishing that convergence. In the system described in U.S. Pat. No. 4,524,307, a technician must access a group of potentiometers and adjust them while observing the display with a visual aid (required to observe the amount of misconvergence) in order to achieve convergence. Besides being physically difficult for the technician, it is time consuming and, therefore, costly. These alignment difficulties are compounded by interaction within the electron gun convergence assemblies which causes the adjustments to interfere with one another, resulting in an iterative adjustment requirement to compensate for such interference.
The present invention solves the prior art difficulties by using a computer to calculate correction waveforms based upon keyboard selected correction coefficient magnitudes. This computed correction data is fed into a non-volatile memory in the display, e.g., via a 1553 port into an electrically erasable read only memory (EPROM). Space and power are decreased using a custom integrated circuit which is made practical through the basic digital design of the invention's convergence correction architecture. The invention takes advantage of the capabilities of this integrated circuit design to increase the processing power of the system with respect to the stored convergence data and, therefore, reduce the storage size and cost over other known digital systems. The storage in memory of specific composite correction waveforms precludes the necessity for on-board correction waveform generators as is disclosed in the above-referenced analog approach. Thus, the circuitry required is greatly reduced.
Because of the digital nature of the convergence waveform generation process of the invention, adjustments for a display can be preset to nominal values before the alignment process begins. The invention uses a digital convergence algorithm whereby interaction between convergence adjustments caused by the electron gun and convergence assembly design are subtracted out before the convergence corrections selected by the aligner are loaded into memory. Thus, from the point of view of the person accomplishing the alignment, there is little interaction between adjustments to various locations on the CRT.
Some convergence control techniques used in the past have implemented digital methods of convergence. For example, U.S. Pat. No. 4,385,259 to Chase, et al. teaches an apparatus for providing precise convergence compensation in a shadow mask type color CRT display. In the Chase device coarse compensation is provided by the coefficients of primary terms X.sup.2, Y.sub.2 of the beam's longitudinal and vertical position polynomial in analog format, and fine compensation is provided by digital programmable read only memories (PROMs). The fine compensation done by Chase is representative of the precise values of the coefficients of the remaining terms of the polynomials, and a digital to analog converter converts the fine compensation to an analog format. The coarse and fine data are then summed together and applied to the convergence correction coils of the CRT. Chase, however, does not address the important problem of aliasing caused by undersampling of correction waveforms inherent in such a digital convergence implementation. That is, adjustments made in one portion of the CRT are not independent using the Chase technique, making the systems very hard to adjust. The invention overcomes this problem by determining the proper sampling requirements for accurate waveform reconstruction. The Chase system resulted in adjustments that are not independent because the sampling requirements for reconstructing waveforms in such a digital system require either large amounts of memory or the application of interpolation techniques as taught in the present invention and not in the prior art. Crosstalk precorrection is an additional advantage of the digital system taught in this invention.